1. Field of the Invention
The invention relates to satellite antennas. More specifically, the invention relates to an inexpensive to manufacture, for example, Satellite Digital Audio Radio (SDAR) antenna having a low profile, for example, suitable for mounting on a motor vehicle.
2. Description of Related Art
SDAR is a form of digital satellite radio, currently offered on a subscription basis by XM and Sirius. SDAR receives in the S-Band frequency range (2.3 Gigahertz Band) with upper hemisphere coverage. To provide reception in urban environments where satellite line of sight signals may be blocked by earth contours, buildings and/or vegetation SDAR uses both satellite and terrestrial mounted transmitters and therefore requires antennas with vertical radiation patterns (satellite) as well as improved low angle performance (terrestrial). XM specifies antenna performance of 2 dBic over a range of 25-60 degrees elevation. Sirius specifies antenna performance of 3 dBic over 25-75 degrees elevation and 2 dBic over 75-90 degrees elevation.
Prior SDAR antennas have used a left hand circular polarized quadrifilar antenna element configuration. Another antenna element configuration used with SDAR is the curved cross dipole configuration. Both types of antenna structures have antenna element vertical heights of over one inch.
SDAR is beginning to have wide use in consumer vehicles where a minimized antenna profile is preferred. Low profile antennas increase resistance to accidental breakage from, for example, automated car washes and tree limbs. Less visually noticeable from a distance, low profile antennas also reduce vandalism and theft opportunities. Also, negative effects on aerodynamics and disruption of vehicle design aesthetics are minimized.
Competition within the antenna industry has also focused attention on minimization of materials and manufacturing process costs.
Circular microstrip antennas have a fundamental TM11 excitation mode with a relatively narrow beam. Circular microstrip antennas have been used for satellite reception where an upper hemisphere radiation pattern with poor low angle coverage is acceptable, for example with Global Positioning Satellites (GPS). Circular microstrip antenna designs are inexpensive, durable and have an extremely low profile. Microstrip antennas may be configured to operate in a TM21 higher order mode that creates a conical radiation pattern with a null at center/vertical, useful for receiving low angle terrestrial originated signals.
Therefore, it is an object of the invention to provide an antenna, which overcomes deficiencies in the prior art.